In a wireless communication system, a base station may provide one or more coverage areas, such as cells or sectors, in which the base station may serve user equipment devices (UEs), such as cell phones, wirelessly-equipped personal computers or tablets, tracking devices, embedded wireless communication modules, or other devices equipped with wireless communication functionality (whether or not operated by a human user).
In general, each coverage area may operate on one or more carriers each defining one or more ranges of frequency spectrum and having a respective “downlink channel” for carrying communications from the base station to UEs and a respective “uplink channel” for carrying communications from the UEs to the base station. Such carriers may be frequency division duplex (FDD), in which the downlink and uplink channels are defined as separate respective ranges of frequency, or time division duplex (TDD), in which the downlink and uplink channels are defined on a common range of frequency but distinguished through time division multiplexing. Further, the downlink channel and uplink channel of each carrier may also be divided into respective sub-channels for carrying particular communications, such as one or more control channels for carrying control signaling and one or more traffic channels for carrying application-layer data and other traffic.
For instance, in a system operating according to an orthogonal frequency division multiple access (OFDMA) protocol, such as the Long Term Evolution (LTE) standard of the Universal Mobile Telecommunications System (UMTS) for example, the air interface is divided over time into frames and sub-frames each defining two slots, and the uplink and downlink channels are each divided over their frequency bandwidth into sub-carriers that are grouped within each slot into resource blocks. When a UE is positioned within coverage of a base station in such a system, the UE may register or “attach” with the base station on a particular carrier on which the base station is configured to provide, and the base station may then schedule particular downlink and uplink resource blocks on that carrier to carry data communications to and from the UE. Further, the base station and UE may modulate their air interface data communications at a coding rate selected based on quality of the UE's coverage, such as with higher rate coding rate when the UE is in better coverage of the base station and with a lower coding rate when the UE is in worse coverage of the base station.
In such LTE systems, a Hybrid Automatic Repeat Request (HARQ) procedure can be used. According to the HARQ approach, after a transmitting entity has transmitted a block of data, the transmitting entity waits to receive a HARQ response from the receiving entity. If the transmitting entity receives a positive acknowledgement (ACK) as the HARQ response, then no re-transmission is needed and the transmitting entity can transmit additional data. If the transmitting entity receives a negative acknowledgement (NACK) as the HARQ response, then the transmitting entity re-transmits the data. The transmitting entity may also re-transmit the data if the transmitting entity does not receive any HARQ response within a certain period of time.
This re-transmission approach can allow data to be successfully transmitted from a transmitting entity to a receiving entity even when there is a substantial probability that the transmitted data will be received with one or more errors, for example, because of poor radio frequency (RF) conditions. Specifically, the data can be re-transmitted multiple times until the data is received without errors. This re-transmission approach, however, also increases latency. For example, there can be a period of delay between when the transmitting entity transmits data and when the transmitting entity receives a NACK response from the receiving entity and another period of delay between when the transmitting entity receives the NACK response and when the transmitting entity begins re-transmitting the data.
To reduce the delay associated with re-transmitting data, LTE supports a bundling option for data transmissions by a UE in the Physical Uplink Shared Channel (PUSCH). Normally, a UE transmits data in one transmission time interval (TTI), which corresponds to a 1 millisecond (ms) subframe, and then waits to receive a HARQ response before re-transmitting the data or transmitting additional data. However, when TTI bundling is used, the UE transmits the same data in multiple consecutive TTIs (i.e., a “bundle” of TTIs) and then waits to receive a HARQ response. In this way, the UE can transmit multiple instances of the same data, which allows for more robust reception of the data, but without the delay that would be associated with the UE transmitting the data multiple times and waiting for a HARQ response after each transmission.
The number of consecutive TTIs used to transmit the same data for a particular communication before waiting for a HARQ response may be referred to as the “bundling size” of the TTI bundling scheme employed to transmit that communication. Conventional LTE systems typically use TTI bundling with a fixed bundling size of four TTIs. However, other bundling sizes are also possible.
In a further aspect of OFDMA protocols, such as LTE, reception at cell edges may be problematic for various reasons. For example, the greater distance to a base station at a cell edge may result in lower signal strength. Further, at a cell edge, interference levels from neighboring cells are likely to be higher, as the wireless communication device is generally closer to neighboring cells when at a cell edge. Accordingly, serving systems (e.g., LTE networks) may use TTI bundling to improve coverage at a cell edge for uplink communications on a single carrier.
In another effort to improve the quality of service at cell edges, 3GPP LTE-A Release 11 introduced a number of Coordinated Multipoint (CoMP) schemes. By implementing such CoMP schemes, a group or cluster of base stations may improve service at cell edges by coordinating transmission and/or reception in an effort to avoid inter-cell interference, and in some cases, to convert inter-cell interference into a usable signal that actually improves the quality of service that is provided.
LTE-A Release 11 defined a number of different CoMP schemes or modes for both the uplink (UL) and the downlink (DL). For the downlink, two basic types of CoMP modes are set forth: joint processing (JP) schemes and coordinated scheduling/beamforming (CSCH or DL-CSCH) schemes. For the uplink, numerous types of CoMP modes have been devised.
Uplink CoMP modes may involve interference rejection combining (IRC) or coordinated scheduling for purposes of reducing or preventing interference between transmissions from different user entities (UEs). Additionally or alternatively, various uplink CoMP modes may involve “joint reception” and/or “joint processing.” Joint reception generally involves multiple base stations receiving an uplink signal that is transmitted by a given UE. Joint processing generally involves the multiple base stations that received the uplink signal from the UE, sending the respectively received signals or a decoded and/or processed version of the respectively received signals to one another, or just to a master base station in the group, such that the multiple received versions of the UE's transmission can be combined to improve reception and/or reduce interference.
Various types of joint processing have been implemented on the uplink. For example, joint processing on the uplink can be centralized. When a centralized CoMP mode is implemented on the uplink, the coordinating base stations may simply pass the entire received signal from a given UE on to a master base station, which then uses the received signals from multiple base stations to decode and/or process the signal from the given UE. Joint processing on the uplink can also be de-centralized to varying degrees. Specifically, when a decentralized CoMP mode is implemented on the uplink, a coordinating base station may decode and/or process the received signal from a given UE, and then send the decoded and/or processed signal from the given UE to the master base station. The master base station can then combine or select from the decoded and/or processed versions of the UE's transmission, which are sent to the master base station from one or more coordinating base stations that receive the UE's signal (and possibly a version of the UE's signal that is received at the master base station itself).